Understanding the development of substance use problems is a vital health concern because of individual health and nationwide economic costs of dealing with substance use related injury, illness, death, crime, law enforcement, and lost productivity1. The research described in this application is designed to examine mechanisms of development of adolescent substance use, with the aim of identifying targets for prevention of substance use. Pathways by which genes, hormones, and family environments operate for the development of substance use are not fully understood, in part because studies have not considered each of these influences together in the same conceptual model. The central hypothesis driving my program of research is that parental negativity is a mechanism modifying the influence of genetic, hormonal, and behavioral risk for substance use problems during adolescence. Specifically, the proposed research will clarify (aim 1) whether the association between parental negativity and adolescent substance use arises because parents respond to adolescents' genetically influenced substance use in such a way that increases parental negativity, or if substance use arises because parents pass on genes and negative parenting, both influencing substance use, or through direct environmental influences. This research will also show (aim 2) whether parental negativity moderates the association between hormone reactivity and risk for substance use (externalizing psychopathology). Together, these studies combined with my previous research will clarify how parenting, hormone, and genetic risk are related in the development of substance use, and examine how parent-child relationships can modify biological influences on risk for substance use problems. Through findings in the research aims, I will refine a conceptual model considering the transactional, developmental nature of genetic, hormone, and family environmental influences on adolescent substance use. The product of this grant will be three manuscripts: two presenting findings from each research aim, and one presenting the refined conceptual model and supporting evidence. My career goal is to conduct interdisciplinary research with the aim to understand the development of substance use. This dissertation research is a first step toward my career goal. To better prepare myself for my career conducting longitudinal research that addresses gene-environment interplay and hormone functioning in the family context for the development of substance use, I am seeking additional training to 1) further develop skills at conceptualizing and understanding substance use as a phenotype, 2) strengthen my understanding of the role of behavioral endocrinology on risk for substance use during adolescence, and 3) fully integrate family environmental influences with genetic and hormone influences in the transactional developmental model of the development of adolescent substance use. Training in substance use, behavioral endocrinology, family processes, and research ethics are integral to completing the research aims, and preparing me for a career in which I am a NIH funded researcher investigating the development of adolescent substance use.